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Biomedical subjects

W M Star

Publications and source records attributed to W M Star.

64 records · Page 4Linked to original sources
PubMed1988

Light dosimetry in optical phantoms and in tissues: I. Multiple flux and transport theory.

This is the first of two papers on the quantitative measurement of light energy fluence rates in optical phantoms and in tissues, in vitro and in vivo. The theory discussed in the present paper will be used in a forthcoming experimental paper to quantitatively check measurements of light energy fluence rates. A simple multiple flux model, which is equivalent to the diffusion approximation, is derived from the equation of transfer in a plane as well as in a spherical geometry. The equations obtained are similar to those of the Kubelka-Munk and related heuristic models. This permits conclusions regarding the limitations of these models and the values of their constants. The heuristic models are equivalent to diffusion theory for diffuse incident light, but not for collimated incident light. We also present a simple calculation of the radiance as a function of direction in the diffusion domain. This, together with the effective attenuation coefficient, permits indirect experimental determination of both the albedo and the anisotropy factor (g) of the scattering function. Similarity relations are discussed, as they result from the so called delta-Eddington approximation, leading to the conclusion that far from boundaries and sources light propagation characteristics do not change very much when g and omega s are varied, provided omega s (1-g) is kept constant (omega s = scattering coefficient). Therefore, only two optical constants are required to approximately describe light propagation in homogeneous and isotropic media in the diffusion approximation.

Mathematics↗
PubMed1987

Light dosimetry: status and prospects.

This paper is a report on the state of the art of light dosimetry in photomedicine and photobiology. The basic quantity of interest is the radiant energy fluence rate, which can either be measured using a suitable probe, or calculated theoretically from measured optical constants. First, theoretical models used to analyse experimental transmission and reflection data are briefly discussed. It is shown that a two-flux model derived from the transport equation in the diffusion approximation resembles the Kubelka-Munk and other heuristic models. This illustrates the limitations of these models and suggests their abandonment in favour of transport theory. For theoretical energy fluence rate calculations at least three optical constants are needed, namely the absorption coefficient, the scattering coefficient and the average cosine of the scattering angle. These three constants have been measured for very few tissues. In principle only two of the three constants can be measured directly on thin samples, independent of a theoretical model. The energy fluence rate can be measured quantitatively with a miniature fibre optic probe with isotropic response. Such measurements allow indirect determination of the three optical constants. It appears that we are just beginning to understand the distribution of light energy fluence rate in tissues. Tasks for the near future are comparison of methods to measure optical constants, quantitative checks of calculated and measured energy fluence rates in model tissues and optical phantoms and further development of theoretical models. Particular attention is required for boundary conditions, with and without refractive index matching.

Animals↗
PubMed1986

Destruction of rat mammary tumor and normal tissue microcirculation by hematoporphyrin derivative photoradiation observed in vivo in sandwich observation chambers.

The effect of hematoporphyrin derivative photoradiation on tumor and normal tissue microcirculation was studied microscopically in vivo on rats with mammary carcinomas transplanted into subcutis in transparent observation chambers. One day after i.p. injection of hematoporphyrin derivative (15 mg/kg), chambers were exposed to red light (632 +/- 2 nm, eight light dose values, 0 to 270 J/cm2). After an initial blanching (ischemia) of the tumor accompanied by apparent vasoconstriction, reperfusion was observed with a slowing down of the tumor circulation, vasodilatation, and eventually a complete stasis, together with diffuse hemorrhages and subsequent necrosis. Besides, in large normal tissue vessels, platelet aggregates were observed, but no hemorrhage. Tumor regrowth occurred unless the tumor circulation and the adjacent normal tissue circulation were both destroyed. Tumor cell viability after treatment was assessed by transplanting the tumor from the chamber into the flank of the same animal. Even after a combined porphyrin and light dose 4 times the lethal dose for all tissues in the chamber, five of five transplanted tumors did regrow. This leads to the conclusion that, in our model system, tumor cell death after photoradiation occurs secondary to destruction of the microcirculation. In order to obtain additional information on normal tissue damage, rat ears were also irradiated. For the same light dose, the biological effect was only slightly larger than that of the normal tissue in the observation chambers, even though the measured ratio of porphyrin concentrations in ears and normal tissue in the chambers (subcutis) was about six.

Animals↗
PubMed1985

Hematoporphyrin derivative photoradiation treatment of experimental malignant melanoma in the anterior chamber of the rabbit.

The effects of Hematoporphyrin Derivative Photoradiation Therapy (HpD-PRT) on Greene's amelanotic melanoma implanted into the anterior chamber of rabbits have been examined by biomicroscopy, fluorescein angiography and histopathology. The tumors were irradiated 24 hours after injection of HpD when both the porphyrin concentration and the porphyrin ratio tumor/iris were highest. Blanching and shrinkage of tumors were the first signs of tumor destruction. Fluorescein angiography as soon as 20 minutes after irradiation found non-perfusion of blood vessels at the tumor surface. Histopathological observation of vessel wall destruction is in agreement with this finding. Subtotal tumor necrosis was demonstrated in 12 out of 13 experiments. Necrosis was complete in only one experiment. Clusters of viable tumor cells were found when shielded behind pigment, at the tumor periphery and around some blood vessels. Lens damage was observed after irradiation when the iris pigment epithelium was disorganized by the tumor. The iris contained high concentrations of porphyrin and PRT resulted in depigmentation, non-perfusion of the capillary bed, damage to larger iris vessels and finally atrophy. Light intensity measurements were performed in vivo during PRT. The average effective attenuation coefficient at 630 nm was 0.56 mm-1 at the beginning of irradiation and 0.87 nm-1 at the end. Results indicate that as a treatment HpD-PRT in itself might be insufficient but may prove to be an effective modality in combination with other tumor destructive therapies.

Animals↗